Cannot send Mail - server error 451

An unknown error has occurred.
Subject 'Re: XXXXXXXXXXXXXXXXX' [I have X'd out the subject]
Server Error: 451
Server Response: 451 Invalid Smtp line - Should end with CRLF
Server: 'mail.XXXXXXXXXXXX.com' [I have X'd out the name of the ISP]
Windows Live Mail Error ID: 0x800CCC6A
Protocol: SMTP
Port: 25
Secure(SSL): No

Windows Mail was working but now error message above appears when SENDING. Will receive but not send from my account. Account details appear to me to be correct. No software changed as far as I know. Security (AVG) kept up to date. OS is Windows 7 Home Premium 64 bit. Laptop is only a few weeks old. Although I have been told error 451 points to ISP's server, the problem does seem to be at my end, and on this laptop as I can still send mails out on my Desktop PC through the same wireless router using the same server details and email account (though using Outlook 2003 on that machine. Thanks for reading this and for any help you are able to give.

I had a similar probleml and figured out what was wrong. I removed the message referenced by the subject of the error and it worked. I suspect the message was formatted in a way that caused it to break.